A thin film transistor (TFT: Thin Film Transistor) has been widely used as a switching device for a display for a liquid crystal display device or the like. A representative TFT has a configuration in which a gate electrode, an insulator layer and a semiconductor layer are stacked in this sequence on a substrate, and has, on the semiconductor layer, a source electrode and a drain electrode being formed with a predetermined interval therebetween. The organic semiconductor layer constitutes a channel part, and an on-off operation is conducted by controlling electric current flowing between the source electrode and the drain electrode by a voltage applied to the gate electrode.
Conventionally, this TFT was fabricated by using amorphous or polycrystalline silicon. However, a CVD (chemical vapor deposition) apparatus used for fabrication of a TFT using silicon is very expensive, and an increase in size of a display or the like using a TFT had a problem that the production cost increased significantly. Further, there was a problem that, since forming amorphous or polycrystalline silicon into a film requires significantly high temperatures, the type of a material which is usable as a substrate is limited, and hence, there were also problems that resin substrates or the like which were light in weight could not be used.
In order to solve the problem, a TFT using an organic substance (hereinafter often referred to as an organic TFT) instead of amorphous or polycrystalline silicon has been proposed. As the film-forming method which is used when a TFT is fabricated by using an organic substance, a vacuum vapor deposition method, a coating method or the like are known. According to these methods, it is possible to realize an increase in size of a device while suppressing an increase in the production cost, and is also possible to allow the process temperature which is required at the time of film formation to be relatively low. Accordingly, in such an organic TFT, there are advantages that only small restrictions are imposed on the type of materials used for a substrate. Therefore, its practical use has been expected and research reports have been actively made.
A practical organic TFT is required to have a high carrier mobility (hereinafter often referred to as the “mobility”), a large on-off ratio of current and excellent storage stability. Meanwhile, the on-off ratio refers to herein a value which is obtained by dividing a current flowing between source and drain electrodes when a gate voltage is applied (ON) by a current flowing between source and drain electrodes when a gate voltage is not applied (OFF). The on-current normally means a current value at the time when the current flowing between source and drain electrodes is saturated (saturation current) after increasing the gate voltage.
As a p-type organic semiconductor material used in an organic TFT, a polymer such as a conjugated polymer or thiophene, metal phthalocyanine compounds, and fused aromatic hydrocarbons such as pentacene or the like have been known. However, an organic semiconductor material which satisfies all of the required performances has not yet been developed.
Of the above-mentioned organic semiconductors, pentacene, which is a polycyclic ring-fused compound, has attracted attention as a material which has a mobility as high as that of amorphous silicon due to its conjugated system, and has been actively studied. However, pentacene has a disadvantage that the stability in the atmosphere is low.
Under such circumstances, as the material which has storage stability and has high carrier mobility, various polycyclic ring-fused compounds have been studied.
For example, a polycyclic ring-fused compound having five rings in which two benzofuran skeletons are fused to the benzene ring was reported in a meeting as an effective compound since it has a carrier mobility of on the order of 2×10−1 cm2/Vs (Non-Patent Document 1). Further, a polycyclic ring-fused compound having four rings in which two furan rings are fused to the naphthalene ring or a polycyclic ring-fused compound having five rings in which two furan rings are fused to the anthracene ring has been proposed as a material of an organic semiconductor layer of an organic TFT. However, no specific examples or the like have been made on a polycyclic compound having 6 rings (Patent Document 1).
Further, the following compounds have been synthesized (Non-Patent Documents 2 to 4). However, the performance as the organic semiconductor material has not yet been demonstrated.
